Immigration is a challenging and often controversial topic. We have written some on the economic benefits and costs associated with the inflows of low-skilled (possibly undocumented) immigrant workers into the United States here
and here. In this macroblog post, we discuss some interesting trends in undocumented immigration.
There are no official records of undocumented immigration flows into the United States. However, one crude proxy for this flow is the number of apprehensions at the U.S. border. As pointed out in
Hanson (2006), the number of individuals arrested when attempting to cross the U.S.-Mexico border, provided by the Department of Homeland Security 
(DHS), is likely to be positively correlated with the flows of attempted illegal border crossings (see chart 1).
The apprehensions series displays spikes that coincide with well-known episodes of increased illegal immigration into the United States, such as after the financial crisis in Mexico in 1995 or during the U.S. housing boom in the early 2000s. Importantly, the series also shows a sharp decline in the flows of illegal immigration at the U.S.-Mexico border during the last recession, and those flows have remained at historically low levels since then.
A better proxy for illegal immigration from Mexico would adjust the number of apprehensions for the intensity of U.S. border enforcement (for example, the number of border patrol officers). The intuition is straightforward: for the same level of attempted illegal crossings, greater enforcement is likely to result in more apprehensions. Chart 2 shows the border patrol staffing levels as an indicator of enforcement intensity.
As the chart shows, the sharp decrease in apprehensions after the Great Recession occurred despite a remarkable increase in border enforcement, indicating that the decline in migration flows in recent years may have been even more abrupt than implied by the (unadjusted) border apprehensions shown in chart 1.
The measure of inflows shown in chart 1 is largely consistent with estimates of the stock of undocumented immigrants in the United States, such as those provided by a
new study by the Pew Research Center based on data from the U.S. Census Bureau. After having peaked at 12.2 million in 2007, the stock of unauthorized immigrants fell during the Great Recession and remained stable afterwards, most recently at 11.1 million in 2014. Also, the composition of this stock has shifted since the Great Recession. Although the population of undocumented Mexican immigrants fell by more than one million from its 6.9 million peak in 2007, the number of undocumented immigrants from Asia, Central America, and sub-Saharan Africa remained relatively steady as of 2014 and even increased in some cases. For example, the population of unauthorized immigrants from India rose by about 130,000 between 2009 and 2014. However, a lot of this type of unauthorized immigration is a result of overstayed visas rather than from people crossing the border without a visa.
What do these numbers suggest about the future? It is likely that the flows of undocumented immigrant labor between Mexico and the United States reflect differences in demographic patterns and economic opportunities between the two economies. In the United States, the baby boom came to an abrupt halt in the 1960s, causing a notable slowdown in the native-born labor supply two decades later. In contrast, Mexico's fertility rate remained high for much longer, hovering at 6.7 births per woman in 1970 versus 2.5 in the United States (see chart 3).
Mexico's labor force expanded rapidly during the 1980s, which, juxtaposed with the Mexican economic slump of the early 1980s, unleashed a wave of Mexican migration to the United States (Hanson and McIntosh, 2010). Also encouraging this flow was the steady U.S. economic growth during the "Great Moderation" period from the mid-1980s up through 2007 (Bernanke, 2004). More recently, however, Mexico's fertility rate has fallen (as in some Central American economies), and economic growth there has mostly outpaced that of the United States. Therefore, it is perhaps not too surprising that demographic trends—along with greater enforcement—have caused the inflows of undocumented migration at the U.S.-Mexico border to slow in recent years. Shifts in demographic and economic factors across countries are likely to continue to influence undocumented immigration in the United States.
Note: The views expressed here are those of the authors and do not necessarily reflect the views of the Federal Reserve Banks of Atlanta or Boston.
By Federico S. Mandelman, a research economist and associate policy adviser in the in the Atlanta Fed's research department, and
